Titanium alloy



United States Patent 3,258,335 TITANIUM ALLOY Allan J. Hatch, Las Vegas, Nev., assignor to Titanium Metals Corporation of America, New York, N.Y., a corporation of Delaware No Drawing. Filed Nov. 12, 1963, Ser. No. 323,066 4 Claims. (Cl. 75-1755) This invention relates to titanium base alloys and more particularly to such alloys which are strong, light and readily fabricated into useful shapes and forms.

Titanium base alloys are advantageously light when aluminum is employed as the principal or sole alloying element. Aluminum, in addition to lowering the specific gravity of titanium, increases its hardness and strength. Aluminum is an alpha stabilizer, that is, tends to stabilize the alpha phase in titanium alloys of which it is a constituent. Alpha-type titanium base alloys are characterized by strength, ductility and weldability which are important properties in aircraft and missile manufacture, for example, and for other purposes where these attributes are particularly desirable. More than a certain proportion of aluminum cannot, however, be employed as an alloying element in titanium base alloys because of its tendency to reduce ductility. Aluminum, when present above about 7 or 8 percent by weight, will affect ductility to the extent that the alloy is embrittled. Even lower percentages of aluminum in the 4.5 to 6 percent range, while not adversely affecting useful ductility, result in directionality and difficulties in working such an alloy into useful shapes and forms. Directionality is important since this involves mechanical properties which are different for diiferent directions across a sheet or strip. Often tensile strength is substantially higher in the direction transverse to the rolling direction than it is parallel to such direction. Such directionality makes further fabrication difficult. To avoid directionality, materials that exhibit this property are often cross-rolled but this is expensive and cannot obviously be employed when continuous strip rolling is employed to produce a flat rolled product.

It is therefore a principal object of this invention to provide an improved titanium base alloy. Another object of this invention is to provide a light, strong, titanium base alloy which can be more readily fabricated. Another object of this invention is to provide a light, strong titanium base alloy which is essential-1y non-directional. Another object of this invention is to provide a light, strong titanium base alloy which is susceptible to texture hardening. These and other objects of this invention will be apparent from the following description thereof.

This invention, in its broad aspects, contemplates a titanium base alloy consisting essentially of by weight from 3.5% to 4.5% aluminum, from 0.1% to 0.4% oxygen, and from 0.15% to 0.4% iron, with the balance titanium and incidental impurities. Preferably, the alloy consists essentially of about 4% aluminum, about 0.3% oxygen with a range of between 0.25% and 0.35%, about 0.2% iron with the balance titanium and incidental impurities.

The aluminum content of the alloy of this invention is important, and the proportions within which it may be present are critical. Less than about 3.5% aluminum will not provide effective strengthening. More than about 4.5% aluminum will provide higher strength but at the same time tend towards reduced ductility, and will adversely affect the directionality properties of the 3.5% to 4.5 aluminum content alloy.

The oxygen content of the alloy of this invention is also important and the proportions within which it is present are also critical. Normally it is difficult to obtain commercial titanium sponge containing less than 3,258,335 Patented June 28, 1966 0.1% oxygen except as specially produced material, and often at a premium price. Utilization, therefore, of titanium sponge containing 0.1% oxygen or higher is extremely desirable from a commercial point of view. Oxygen, as is well known, acts somewhat like other alloying elements in that it tends to strengthen titanium base alloys when present up to a certain critical percentage, -but above this amount seriously impairs ductility, often to the extent resulting in embrittlement. In the alloy of this invention, oxygen is present in amount from 0.1% up to about 0.4%, a greater amount than this being undesirable because of its embrittling tendency. Less than 0.1% oxygen will not impart any appreciable strength increase. It should be noted that the 0.4% upper limit to the range of oxygen in the alloy of this invention is somewhat higher than normally considered desirable for other type titanium base alloys. The advantageous presence of this higher percentage of oxygen in the alloy of this invention apparently is made possible by the conjoint presence of a small proportion of iron which uniquely tends to promote ease of rolling and fabrication and may therefore offset otherwise deleterious effects from a somewhat higher than normal oxygen content.

The presence of iron in the alloy of this invention is also important and the proportions within which it may be present are like the aluminum and oxygen content, critical. Less than 0.15% iron will not produce the advantageous effect that the higher percentages within the range defined according to this invention will produce. Small amounts of iron, below the range of this element in the alloy of this invention, are often considered to be in the incidental impurity range and in such amounts, iron is not considered to produce any manifest effect on the mechanical properties of the alloy. I have discovered, however, that iron in amount between 0.15% and 0.4%, when present in a titanium base alloy, together with the particular amounts of aluminum and oxygen specified, will, quite unforeseeably, promote ease of fabrication, and particularly ease of rolling, to produce sheet and strip from billets and ingots, and, in addition, the material so produced will be characterized by lack of directionality. At least 0.15% iron is necessary to achieve such results and more than 0.4% iron will not provide any additional benefit and, in addition, will result in the presence of an appreciable amount of a powerful beta stabilizer which will tend to adversely affect the essentially all-alpha properties of the alloy of this invention.

Incidental impurities may be present in the alloy of this invention in aggregate and individual amounts such that they will not materially affect the essential nature of the alloy and its properties as herein described. Such impurities may be present in the titanium sponge and alloying materials employed to produce the alloy or may be picked up, for example, as atmospheric contamination, during processing. The total of incidental impurities should generally not exceed about 0.25 and the interstitials carbon and nitrogen should not total more than about 0.15

The alloy of this invention may be produced by any method by which the titanium and alloying elements are melted together to form a substantially homogeneous alloy composition. Preferably, titanium sponge of required purity and particularly with respect to its oxygen content, is admixed with subdivided aluminum and 'iron in proper amounts and the mixture compressed into compacts. Alternatively, if desired or more convenient, the required oxygen content of the alloy may be obtained by incorporating an oxygen-containing compound such as Ti0 into the compact mixture. Also if more convenient or desirable, the proportion of iron desired may be introduced as a constituent of the titanium in required amount to produce the percentage required. After pressing the mixture into compacts, these are welded together to form an electrode which is preferably melted in a consumable electrode arc melting furnace to produce an ingot of alloy. The so-produced alloy ingot may itself be employed as an electrode in a subsequent remelting step to provide improved homogeneity in a final alloy ingot.

The following describes a selected embodiment of the practice of this invention.

Example 1 An alloy was produced with nominal composition as follows:

4% Al-0.25% O -0.25% Fe To produce this alloy, titanium sponge of commercial purity and of 117 Brinell hardness was admixed with suflicient pure aluminum shot and pure iron nails to provide the required percentages of aluminum and iron in the alloy. The oxygen content of the alloy was provided by introducing TiO powder in amount to provide 0.25% oxygen in the alloy ingot. These ingredients were carefully mixed and then compressed into compacts which were welded together to produce a consumable electrode. This electrode was melted in an arc furnace to provide a first melted ingot which was itself employed as a consumable electrode in a second melting step to produce a remelted homogeneous alloy ingot.

The so-produced alloy ingot was surface machined to remove outer porosity, and impurities, and the conditioned ingot was forged at 2050 F. to form a 3" x 3" billet. This billet was cut into slabs 3" x 3" X 1%" and one of these slabs was upset rolled hot at 1775 F. to 0.8" sheet. This sheet was further rolled to 0.15" thickness at 1650 F. and after intermediate anneal, sand blasting and pickling, was finally cold rolled to 0.060 gauge and finally annealed at 1400 F. and air cooled. From the finished 0.060" gauge alloy sheet, specimens were cut for tensile testing according to A.S.T.M. standard procedures and also for determining R values as hereinafter described in more detail. Average properties determined on the 0.060" gauge sheet are shown below:

It should be noted that the strength of the alloy is good and ductility is good based on an elongation of around 21 or 22%. It should also be noted that the properties in the longitudinal and transverse directions are substan tially identical, indicating that the allow has little, if any, directionality.

The alloy of Example 1 is light, having a specific gravity of 0.160 lb./in. compared to the specific gravity of 0.163 lb./in. for pure titanium.

The alloy of this invention, when plastically deformed to produce a specific texture, has been found to possess extremely high strength when subject to biaxial stress. The alloy after fabrication, that is rolling or otherwise forming into sheet, plate or other products, will be characterized by a particularly texture resulting from an orientation of the crystal lattice basal planes substantially parallel to the surface of the plastically deformed alloy. Such texture is different from that ordinarily obtained by plastic deformation of pure titanium and most other alpha titanium alloys. Ordinarily, such texture is characterized by an orientation of the basal planes at an angle between 27 and 40 in the direction transverse to the direction of deformation which could be, for example, the direction of rolling. It is postulated that .this inclinati n 913 the basal planes in such other alloys,

when plastically deformed, is an eifect caused by twinning on pyramidal planes resulting in rotation and tilting of the crystal basal planes. The reason for level orientation and lack of rotation and tilting of the crystals in the titanium-aluminum-oxygen-iron alloys of this invention is not well understood but is presumed to be some peculiarity in the crystal structure which prevents or inhibits twinning. But whatever the reason, it has been found that the strength under biaxial stress of the textured alloy of this invention can be as much as 50% greater than would be expected and compared to other titanium alloys of tilted texture.

Examination of titanium alloy bodies to determine texture is tedious and complicated involving determination of pole figures by X-ray analysis. A much more readily obtained, and for ordinary purposes quite practical, indication can be obtained from the R.value.

The value, R, is an anistropic parameter derived from the original work of Hill 1 and shown by Backofen 2 to have considerable significance on the flow and fracture strength of a material under biaxial tension. Generally, if R has a value greater than 1, there will be a strengthening effect in biaxial tension. Specifically, in a stress field of a cylindrical pressure vessel where hoop stress is equal to twice the stress in the direction of the cylinder axis, an R value of 4 will theoretically increase the yield strength of the pressure vessel by nearly 50% of the yield strength obtained in uniaxial tension.

From the work of Hill it can be shown that R may be determined from a simple uniaxial tension test on a sheet specimen by measurement of strain in the thickness and width directions. Denoting these strains in the usual manner, R is calculated by the following equation:

where w and w are the initial and final width and t and t are the initial and final thickness.

To determine R values as employed according to this invention, 20 micrometer measurements of width and thickness can, for example, be made prior to a uniaxial tensile test and at the limit of uniform elongation.

R value'determined on the same 0.060" sheet sample for which mechanical properties are shown in Table 1 above, showed values from 4 to 6 which represents a substantial degree of texture hardening.

The properties of the alloy of this invention will vary only to a minor extent with changes of content of alloy elements within the described ranges, and the properties shown in Table 1 can be taken as generally typical. The aluminum content is held to a fairly close limit for this element and variation within the 3.5% to 4.5% range will result in slightly higher strength for the higher aluminum content. As the oxygen content approaches the higher limit specified, that is 0.4%, it will be found that strength will be slightly increased, and ductility tends to be reduced. A minimum of 0.15% iron will provide improvement in rollability and this will be obtained throughout the recited range of 0.15% to 0.4%. Over 0.4%, as previously described, the beta stabilizing effect may become undesirable.

The remarkable function of the iron content of the alloy of this invention in improving rollability cannot be explained by application of known metallurgical principles. It does function efficiently in this respect, however, and tests have positively demonstrated a substantial improvement when from 0.15% to 0.4% of iron is pres- Hi11, R., A Theory of the Yielding and Plastic Flow of Anistropic Metals, Proc. Roy. $00., 1948, Ser. A., Vol. 193, p. 281: The Mathematical Theory of Plasticity, Oxford; Clarendon Press, 1950, Chapter XII.

Backofen, W. A., Hosford, W. F., 3'12, and Burke, J. IL, Tgr tinre Hardening, Trans. ASM, March 1962, Vol. 55, P.

ent in the alloy. For example, an alloy consisting of 4% aluminum, 0.25% oxygen and 0.10% iron, balance titanium, could be cold rolled to produce a maximum reduction of only 30% without edge or internal cracking. A similar alloy, produced and fabricated similarly but containing 0.25% iron instead of 0.10%, could be cold rolled to a maximum reduction of 40% without edge or internal cracking. The improvement from 30% to 40% is substantial. The presence of iron as described will also provide easier hot rolling, and the principal benefit from this effect, and which is important, results in the possibility of using a lower hot rolling temperature to obtain an equivalent deformation.

It is surprising also that the alloy of this invention exhibits uniform properties in both longitudinal and transverse directions. This, for some unknown reason, appears to be related to the aluminum content. Tests with a similar alloy containing 6% aluminum instead of 4%, showed higher strength (as would be expected) but a yield strength in a typical specimen of 115,300 p.s.i. in the longitudinal direction compared to 121,200 p.s.i. in the direction transverse to the rolling direction.

The alloy of this invention is useful where its strength, light weight and other mechanical properties are particularly desirable. It may be employed in aircraft manufacture and also for various parts in jet engine construction. When texture hardened, it can be employed for production of pressure vessels, for example, in missile and rockets wherein it will be found to perform capably at normal as well as cryogenic temperatures. It will be found useful, also, for the other applications where light weight and strength and also the characteristic corrosion resistance of titanium and its alloys are particularly advantageous such as, for example, production of hand tools.

I claim:

1. A titanium base alloy characterized by an essentially all alpha microstructure and consisting essentially of by weight;

(a) from 3.5% to 4.5% aluminum,

(b) from 0.1% to 0.4% oxygen,

(c) from 0.15% to 0.4% iron, and,

(d) balance titanium and incidental impurities.

2. A titanium base alloy characterized by an essentially all alpha microstructure and consisting essentially of by weight;

(a) about 4% aluminum,

(b) about 0.3% oxygen,

(0) about 0.2% iron, and,

(d) balance titanium and incidental impurities.

3. A titanium base alloy characterized by an essentially all alpha microstructure and consisting essentially of by weight;

(a) about 4% aluminum,

(b) from 0.1% to 0.4% oxygen,

(0) from 0.15% to 0.4% iron, and

(d) balance titanium and incidental impurities.

4. A titanium base alloy characterized by an essentially all alpha microstructure and consisting essentially of by weight;

(a) from 3.5% to 4.5% aluminum,

(b) from 0.25% to 0.35% oxygen,

(0) from 0.15% to 0.4% iron, and,

(d) balance titanium and incidental impurities.

References Cited by the Examiner UNITED STATES PATENTS Re. 24,013 5/1955 Jalfee --175.5 2,865,742 12/1958 Gullet 75175.5

DAVID L. RECK, Primary Examiner.

W. C. TOWNSEND, C. N. LOVELL,

Assistant Examiners. 

1. A TITANIUM BASE ALLOY CHARACTERIZED BY AN ESSENTIALLY ALL ALPHA MICROSTRUCTURE AND CONSISTING ESSENTIALLY OF BY WEIGHT; (A) FROM 3.5% TO 4.5% ALUMINUM, (B) FROM 0.1% TO 0.4% OXYGEN, (C) FROM 0.15% TO 0.4% IRON, AND, (D) BALANCE TITANIUM AND INCIDENTAL IMPURITIES. 